Most materials exhibit
a change of resistance with applied magnetic fields (magnetoresistance)
that is one of the most common transport measurements made at NHMFL. Magnetoresistance
measurements contain information about a material's electronic structure,
carrier mass, electron-phonon coupling, etc. 4 wires need to be attached
to a sample to measure it's magnetoresistance. Ordinarily, resistance
is measured by placing two probes of an ohm-meter across the ends of whatever
is being measured.

2 wire measurements
do not work very well for samples located in our experimental set-up because
the samples are located at the bottom of a 1.5 meter long probe. The leads
connecting the sample to the outside world have a resistance of ~25W.
This makes it impossible to measure accurately the resistance of just
the sample using the 2 lead measurement method if the sample resistance
is less than 25W. The 4 wire resistance measurement method solves this
problem.

For the 4 wire method,
we pass a current through the sample (2 contacts), then measure the potential
difference across the sample (2 more contacts). We then know that R =
V/I between the voltage leads, which gives us just the resistance of the
sample without the leads and contacts.